During the last 20 years, various manufacturers have introduced a number of web formers operating by the twin-wire principle, a review of said formers being published, e.g., in the journal Pulp & Paper, September 1982.
With increasing running speeds of paper machines, several problems in the web formation have been manifested with more emphasis. In the former of a paper machine, the phenomena that affect the fiber mesh and the water, which is still relatively free in connection with the fiber mesh, such as centrifugal forces, are, as a rule, increased in proportion to the second power of the web velocity. The highest web speeds of the present-day newsprint machines are of an order of 1200 m/min. However, newsprint machines are being planned in which a web speed of up to about 1700 m/min is aimed at.
In the assignee's FI Patent Application No. 904489 (filed Sep. 12, 1990), a twin-wire web former is described in the area of whose forming gap there is a first forming roll, on which the twin-wire zone is curved within a certain sector, which is followed by plane dewatering units or unit. These units comprise a press-support unit, which guides the wire that enters into contact with said unit as a straight run, as well as a dewatering equipment placed facing the press and support unit and provided with a suction and foil equipment, said dewatering equipment removing a substantial amount of water out of the web. The magnitude of the twin-wire turning sector placed in connection with the first forming roll is in the range of about 5.degree. to about 120.degree., preferably within the range of about 35.degree. to about 55.degree..
More particularly, the present invention is related gap formers in which the forming gap is defined between two breast rolls or equivalent turning members, such as turning bars, placed side by side, which breast rolls or equivalent do not operate as forming rolls, at least not to a substantial extent. After the breast rolls, the opposing wires that define the forming gap run as substantially straight runs, while approaching each other, onto the next forming member, which consists of a forming shoe provided with a curved guide deck.
With respect to the prior art related to the present invention, reference is made further to the U.S. Pat. No. 4,769,111 of A. Ahlstrom Corporation, to the assignee's FI Pat. Appl. No. 885609, as well as to the FI Patent Applications Nos. 884606 and 885607 of Valmet-Ahlstrom Inc., in which formers marketed under the trade mark "MB-Former" are described.